1. Field of the Invention
This invention relates generally to digital cordless telephones. More particularly, it relates to a digital communication technique providing increased range and/or voice quality capability within a two channel scheme, e.g., as defined by the DECT standard.
2. Background of Related Art
Cordless telephones have gained in popularity over the years, and can now be found in many if not most homes or businesses. A cordless telephone is one in which the handset is not wired to its base unit, but instead uses wireless communication techniques between a remote handset and its base unit, typically allowing the remote handset to be used up to 1000 feet or more away from its base unit.
Initially, wireless communications between an analog remote handset 502a and its base unit 504a communicated an underlying voice signal in an analog form, as depicted in FIG. 3A. However, the quality of the voice signal in an analog cordless telephone tends to degrade as the distance between the remote handset and its base unit increases, and/or as environmental conditions worsen. Unfortunately, particularly in analog systems, any degradation in the voice signal is perceived and heard by the user of the analog cordless telephone, potentially causing uncomfortable sounds to the user.
Later, digital communication techniques were implemented in the communication path between a digital remote handset 502b and its base unit 504b of a cordless telephone as depicted in FIG. 3B. These cordless telephones are commonly known as digital cordless telephones. The digital communication techniques greatly improved the quality of voice communications using cordless telephones, particularly when the remote handset is used at a distance from its base unit, and/or in the presence of undesirable environmental conditions. In a digital cordless system, incremental degradation in the received signal does not affect the perceived voice quality, and occasional errors in voice data can be corrected using conventional error correction techniques. Thus, a user is provided with consistent quality voice communications generally for the entire range of operation, with environmental conditions potentially decreasing the range of operation of the digital cordless telephone.
Digital cordless telephones present the best opportunities for the future, and continue to be improved and standardized. For instance, digital cordless telephones have more recently begun to adopt a wireless digital communication standard known as the Digital Enhanced Cordless Telecommunications (DECT) for the communication of voice data between a DECT digital remote handset 502c and its base unit 504c, as shown in FIG. 3C.
The DECT standard has been in development by the European Telecommunications Standard Institute (ETSI) since the late 1980""s. Initially, the DECT standard was adopted for use in more complex products such as cordless private automatic branch exchange (PABX) systems. However, today the DECT standard is used to address the needs of a much higher volume market with, e.g., cordless telephones.
The DECT standard can be used to provide wireless access in both indoor and outdoor environments, with cell radii ranging from 50 to several hundred meters. Hence, the DECT standard has been found to be suitable for the development of residential, business and public applications.
The DECT standard is an access system, without any specific (defined) backbone network. As such it relies to a large extent on the backbone network(s) it is attached to. The DECT standard requires an adaptation to a backbone system, and this xe2x80x9cadaptationxe2x80x9d preferably does not form a bottleneck.
Many backbone networks for DECT specific internetworking standards have been developed, e.g. ISDN and GSM. The DECT standard is capable of offering services with these networks, and essentially can be considered to become a part of the backbone network into which it is adapted. Consequently, the DECT standard is no longer just a simple extension of the backbone network in which it is implemented, but rather enhances the services and capabilities of its backbone network.
The interworkings of the DECT standard with many kinds of backbone networks are specified in the DECT standard. Applications defined in the DECT data services profiles include, e.g., Ethernet and Token Ring LANs, TCP/IP, MANs, Modems, RS-232, X.25, Real-Time Video, ATM, Multimedia, Paging, Telemetry, Fax, E-mail, WWW, and X.400.
The general characteristics of the DECT standard include a frequency range of 1.88-1.90 GHz, a carrier spacing of 1.728 MHz, peak transmit power of 250 mW, and 2xc3x9712 channels/carrier. The DECT standard specifies, a duplex method of TDD using 2 slots on the same RF carrier. The communication channels may utilize time division multiple access (TDMA) or frequency division multiple access (FDMA) techniques. Up to 120 duplex channels may be implemented. The specified frame length is 10 milliseconds (mS), the specified speech coding is 32 kbits/s (kb/s) using adaptive differential pulse code modulation (ADPCM) techniques. The specified modulation is Gaussian Frequency Shift Keying (GFSK), and the specified gross data rate is 1.152 Mbit/s.
The conventional implementation of the DECT standard for use in conventional cordless telephones is shown in FIG. 4.
In FIG. 4, the DECT standard includes a physical layer (PHY) 202, a medium access control (MAC) layer 204, a data link control (DLC) layer 206, a network (NWK) layer 208, a user data layer 210, and an application (i.e., control data) layer 212. The general descriptions of each of these layers is as follows.
The physical layer (PHY) 202 typically specifies radio parameters such as frequency, timing and power values, bit and slot synchronization, and transmitter and receiver performance data. FIG. 5 shows the frame structure of the time division multiple access/time duplex (TDMA/TDD) transmission scheme defined by the DECT standard. In general, in each direction in the wireless communication link of a DECT digital cordless telephone, one (1) timeslot is utilized to transmit voice data, and one (1) timeslot is utilized to transmit control data.
Referring again to FIG. 4, the medium access control (MAC) layer 204 is specified in three groups of MAC services. These are the broadcast message control service, the connectionless message control service, and the multi-bearer control service. The medium access control (MAC) layer 204 also specifies the logical channels, which are used by the above mentioned services, and how they are multiplexed and mapped on to the physical channels.
The data link control (DLC) layer 206 specifies whether or not the DLC service operates in acknowledged or unacknowledged mode. The data link control (DLC) layer 206 is also defined to include the provision of addressing, frame delimiting, error control, flow control, segmentation of network layer information fields, fragmentation of DLC frames, and connection handover.
In the user data layer 210, the transparent unprotected service, the frame relay service, the frame switching service, and the rate adaption service are defined.
The network (NWK) layer 208 specifies functions for the link control, call control, supplementary services, connection oriented message services, and mobility management. The network (NWK) layer 208 includes procedures, messages, and information elements for each of these groups.
The application layer 212 includes control data relating to the specific application. For instance, the application layer 212 may include the type speech coding and the necessary functions to provide real-time two-way speech conversation. It defines the speech encoding algorithm and the detailed speech performance characteristics such as sensitivity, frequency response, sidetone, terminal coupling loss, distortion of gain with input level, out of band signals, noise, acoustic shock, delay, and network echo control.
The DECT standard also outlines that a 3.1 KHz telephony teleservice conveyed over a DECT link (including FP and PP) which is capable of being connected (directly or indirectly) to the public network access point shall comply with the requirements in Part 7. However, tethered, fixed point local loop applications are not required to comply with the requirements of the Part 7 standard (e.g., the communications between a remote handset and its base unit in a digital cordless telephone). The DECT standard provides that DECT implementations capable of voice transmission that are not required to comply with the Part 7 standard shall not use more than 20% of the available physical channels for connections that use more than one full slot duplex bearer (i.e., not more than two (2) channels in either direction between the remote handset and its base unit).
FIG. 5 shows the frame structure of the time division multiple access/time duplex (TDMA/TDD) transmission scheme defined by the DECT standard.
FIG. 6 depicts half of the channels used by one DECT digital cordless telephone, i.e., either the two channels used to transmit information from the remote handset to the base unit, or the two channels used to transmit information from the base unit to the remote handset.
As depicted in FIG. 6, within each frame defined by, e.g., a frame pulse 600, one transmission channel contains control data 602 and another transmission channel contains voice data 604.
Various countries, including the TIA in the United States, have adopted a modified version of the DECT standard (e.g., the PWT standard). According to these implementations of the DECT standard, the control data transmitted in the application layer 212 is defined to be sent using error correction techniques provided using a cyclic redundancy check (CRC).
Unfortunately, as a remote handset reaches the full extent of its communication capabilities and the communication signal becomes weak, (e.g., at the end of its usable range in the so-called end of range condition), noise and/or interference often degrade the voice quality of the connection between the remote handset and its base unit. Moreover, if using only CRC checking techniques to correct errors in the control channel, particularly when the digital cordless telephone is in an end of range condition, uncorrectable errors may nevertheless occur in the control data transmitted in the application layer 212. These errors in the control information passed between the remote handset and its base unit may cause serious problems, including the possibility of leading to a dropped call.
Accordingly, there is a need for a more reliable technique for communicating control data between a remote handset and its base unit in a digital cordless telephone utilizing the DECT standard, particularly when the digital cordless telephone is in an end of range condition.
In accordance with the principles of the present invention, a digital wireless communication protocol for use with cordless telephones comprises a control data channel adapted to contain control data relating to a use of a digital cordless telephone. A voice data channel is adapted to contain voice data relating to a conversation using the digital cordless telephone as well as a redundant copy of at least a portion of the control data.
A method of providing diversity for control data in a two channel system including voice data in accordance with another aspect of the present invention comprises communicating the control data in a first one of two communication channels, and communicating encoding voice data in a portion of a second one of the two communication channels. A redundant copy of the control data is combined in a remaining portion of the second one of the two communication channels.